Uranium recovery process



traction treatment is readily filterable.

rates URANIUM RECQVERY PROCESS No Drawing. Application July 13, 1951, Serial No. 236,707

6 Claims. (Cl. 23--14.5)

This invention relates to a process of recovering uranium from very low grade uranium containing ores and ore residues, and more particularly it relates to the selective hot ;acid extraction of uranium from the low grade uranium containing hydroxide precipitates which are obtained by alkali neutralization of sulfuric acid leach liquors derived from the aforesaid low grade uranium containing ores and ore residues followed by a selective precipitation of uranium from the acid extracts derived from the low grade uranium containing hydroxide precipitates. These selective hot acid extraction and selective precipitation steps serve to upgrade the low grade uranium containing hydroxide precipitates and to convert them into high grade uranium concentrates.

The very low grade uranium containing ores and ore residues contain silicates and aluminum and iron containing minerals, in addition to minute amounts of uranium,

and therefore the sulfuric acid leach liquors derived from these ores and ore residues contain large amounts of dissolved silica, iron and aluminum in addition to uranium. The addition of an alkali such as caustic calcined magnesia results in the formation of precipitates which contain large amounts of silica, iron and aluminum, in addition to a small amount of uranium. This leads to the problem of upgrading these precipitates in order to eliminate the silica, aluminum, and iron, and thereby to secure high grade uranium concentrates.

This invention has an object to provide a process for obtaining high grade uranium concentrates from very low grade uranium containing ores and ore residues. A further object is to provide a process for upgrading the low grade uranium containing hydroxide precipitates which are obtained by alkali neutralization of sulfuric acid leach liquors derived from the aforesaid low grade uranium containing ores and ore residues. A still further object is to provide a process for selectively extracting uranium from these low grade uranium containing hydroxide precipitates wherein the undissolved residue from the ex- Another object is to selectively precipitate uranium from the uranium containing extracts derived from these low grade uranium containing hydroxide precipitates. Other objects will appear hereinafter.

These objects are accomplished by the following invention in accordance with which low grade uranium containing hydroxide precipitates which also contain large or dolomite to these sulfuric acid leach liquors, therefore,

amounts of hydrated silicaand iron and aluminum hydroxform. The uranium is then selectively precipitated from the digest liquor. by the addition of hydrogen peroxide thereto. These selective extraction and precipitation steps constitute the most important features of applicants novel 2 process of upgrading low grade uranium containing hydroxide precipitates to obtain a high grade uranium concentrate therefrom.

This invention isgenerally applicable for the upgrading of low grade uranium containing hydroxide precipitates derived from a wide variety of very low grade uranium containing ores, and has been found to be especially useful for selectively upgrading the uranium content of low grade hydroxide precipitates which are derived from sulfuric acid leach liquors obtained from ores that assay less than 0.2% in UaOs. The ores to which this invention is particularly applicable, and on which the greater part of the experimental work described in this specification has been performed, are all mineralogically similar being composed of quartz pebbles surrounded by a quartzitic matrix in which the valuable minerals occur. The ores typically contain about 10 percent of foliated silicate minerals, which may be either sericite or pyrophyllite or both. In addition they contain pyrite, uraninite, carbonaceous material, gold, and very small quantities of heavy minerals such as ilmenite, rutile, chromite, magnetite, zircon, and tourmaline. The pyrite content of these ores varies from about 1 percent to about 3 percent. The pyrite is rich in gold, but does not have much uraninite associated with it. The carbonaceous material in these ores is a fragile hydrocarbon mineral of low specifie gravity which is rich inboth gold and uraninite. Uraninite is the only uranium mineral of importance in these ores. It occurs free and as locked particles with other mineral constituents of the ores, especially hydrocarbon. The gold content of these ores may vary from about 0.005 oz. to about 3.4 oz. per ton. The UsOa content of these ores may vary from about 0.001% to about 0.17% depending upon richness of vein and selectivity of mining.

Instead of processing the ores as they are mined and crushed, it is frequently preferred to apply the invention to ore residues which remain after the ores have been leached with a cyanide solution to extract the gold from the ore. These cyanided residues frequently contain about 1% of metallic iron, weathered aluminum silicates, and some lime derived from the cyanide leaching .in addition to the minerals mentioned in the preceding paragraph. These cyanided ore residues generally assay from 0.015% to 0.030% in U308 from 2% to 3% in Fe, from to 88% in SiOz, from 7% to 9% in A1203, from 1% to 4% in CaO, from 1% to 2% in MgO.

This invention is generally applicable for upgrading the low grade uranium containing hydroxide precipitates which are made by treating sulfuric acid leach liquors of the previously described ores and ore residues with a precipitating agent, such as caustic calcined magnesia or caustic calcined dolomite. These precipitates usually analyze from 1% to 5% in U308, from 5% to 15% in Fe, from 10% to 30% in A1203, from 5% to'-20% in SiOz,

from 5% to 15% MgO, and from 5% to 20% in Get). a

The invention is illustrated but not limited by the following examples.

Example 1 A cyanided ore residue of the type described above which analyzed 0.022% in U308, 2.4% in Fe, 85.4% in SiO2, 7.7% in A1203, 0.6% in CaO, and 1.02% in M50 and from which substantially all of the gold had been extracted by cyanide leaching was agitated at about 20% solids with a solution containing 39 pounds of sulfuric acid (sp. gr. 1.84) and 7.5 pounds of manganese dioxide per ton of ore residue. A leach liquor was thus prepared which had a pH of 3.5 and contained about 0.1 gram of U308, 4 grams of iron, 2 grams of aluminum and '1 gramof silica per liter. The leach liquor thus obtained was acidified to pH 1.5-2.0 and further reacted with addi- 3 tional Mn02 to convert the iron dissolved therein to the ferric state. This oxidized leach liquor was then neutralized with caustic calcined magnesia to a pH of 3. This caused the ferric iron to precipitate. This ferric precipitate, which contained very little uranium, was filtered off, and the filtrate was further neutralized to a pH of 6.5 with more caustic calcined magnesia. The precipitate thus obtained assayed 4.-18% in U303, and approximately 25% in A1203, 15% in Si02, 2% in Fe, 7% in CaO and 10% in MgO.

A wet uranium containing precipitate having the dry assay set forth in the last sentence of the preceding paragraph was converted to a slurry containing about 10% solids and having a pH of 3.5 by the addition of sulfuric acid thereto. This slurry was heated under a reflux condenser for 23 hours at 100 C. At the end of 23 hours .of heating at a pH of 3.5, it was found that 97.9% of the uranium had been extracted from the digested precipitate, and that the digest liquor could be rapidly separated from the digested precipitate by filtration.

Another sample of this same precipitate which was digested for 23 hours at a pH of 3 instead of a pH of 3.5, as described in the preceding paragraph, produced asolution containing 98.1% of the uranium, but the filtering characteristics of this digested slurry were somewhat inferior to that of the digested slurry described in the preceding paragraph.

1.73% in MgO was leached with sulfuric acid in the presence of manganese dioxide, and to the pregnant leach liquor thus obtained there was added caustic calcined magnesia until a pH 6.5 was attained. A precipitate was thus obtained which analyzed 1.94% in U308, 8.5% in Fe, 7.9% in Si02, 17% in A1203, 5% in CaO, 14.2% in MgO, and 0.64% in Mn. The aforementioned precipitate contained both ferrous and ferric iron. An aqueous slurry of this precipitate was aerated for 24 hours while maintaining the pH thereof at about 6 by the addition of calcined dolomite thereto in order to convert all of the ferrous iron in the precipitate to the ferric state.

The slurry of aerated precipitate was mixed with enough sulfuric acid to reduce its pH to 3.4 and to reduce the solids content of the slurry to about 10%. The slurry was then digested with the added sulfuric acid at 90 C. for 1 hour. The temperature of 90 C. was maintained r by boiling the slurry under reduced pressure. The digest Samples of a similar uranium containing precipitate were digested at 100 C. with sulfuric acid for periods ,of 1, 2, 4, and 7.25 hours at pHs of 3.7, 3.7, 3.8, and 3.6 respectively, and uranium extractions of 96.4%, 97.2%, 94%, and 99.6% respectively were thus obtained. All

of these digested slurries filtered rapidly, but the one which had beenv digested for 7.25 hours at 100 C. filtered very fast.

Example 2 A wet low grade uranium containing hydroxide prearated from the digested solid residue by filtration. The

aqueous digestion liquor contained 90.2% of the uranium values that were contained in the precipitate and 9.8%

If the uranium values remained in the digested solid resi- To the aqueous digestion liquor, there was added more than a suflicient quantity of a 30% hydrogen peroxide solution to precipitate all of the uranium dissolved therein. As a'result of this addition of H202 a precipitate of uranyl peroxide was obtained which contained 86% of the uranium values that were originally present in the sample of precipitate that was digested. The uranyl peroxide precipitate was separated from the liquor in which it was precipitated by decantation. The decanted liquor contained only 3.8% of the uranium values that were originally present in the sample of precipitate that was digested. The uranyl peroxide after being dried at 100 C. was found to assay 61% in U308.

It was obvious from the foregoing that the hot sulfuric acid digestion described in the first paragraph of this example coupled with the uranyl peroxide precipitation described in the second paragraph of this example pro- ;vided an expeditious way of obtaining a high grade uranium concentrate from the low grade uranium containing hydroxide precipitate which was digested.

in Fe, 85.5% in Si02, 7.4% in A1203, 1.9% in CaO, and

.90" C. for one hour. .maintained by boiling the slurry under reduced pressure.

liquor was separated from the digested solid residue by filtration and found to contain 97.9% of the uranium values that were originally present in the precipitate that was digested. The digested solid residue retained only 2.1% of the uranium values that were originally present in the sample of precipitate that was digested.

To the aqueous digest liquor there was added enough of a 30% aqueous solution of hydrogen peroxide to precipitate the uranium therefrom. After settling for two hours the precipitated uranyl peroxide was separated by decantation from the liquor in which it was precipitated. The precipitated uranyl peroxide contained 83.1% of the uranium values that were originally present in the sample of precipitate that was digested. After being dried at 100 C., this uranyl peroxide precipitate analyzed 67.52% in U308.

Example 4 rous and ferric iron. An aqueous slurry of this precipitate was aerated for 24 hours while its pH was maintained at about 6 by the addition of calcined dolomite thereto. This aeration converted all of the ferrous iron in the precipitate to the ferric state.

The slurry of aerated precipitate was mixed with enough sulfun'c acid to reduce its pH to 3.5 and to reduce the solids content of the slurry to about 10%. The slurry was then digested with the added sulfuric acid at The temperature of C. was

The digest liquor was separated from the digested solid residue by filtration and was found to contain 91.9% of the uranium values that were originally present in the precipitate that was digested. It was found that 8.1% of the uranium, 80.6% of the aluminum, 99.2% of the .silica and 92.6% of the iron that were originally contained in the precipitate that was digested were retained in the digested solid residue. This showed that the sulfuric acid digestion at 90 C. and at a pH of 3.5 served to selectively extract the uranium values while it left the greater part of the aluminum, silica, and iron with the .digested solid residue.

uranium values that were originally present in the sample of precipitate that was digested. After being dried at 800-1000 0, this uranyl peroxide precipitate analyzed 84.8% in P308.

Example An ore residue which assayed 0.021% in U308, 2.3% in Fe, 87.2% in Si02, 7% in A1203, 4% in CaO, and 0.94% in MgO was leached with sulfuric acid in the presence of sufficient manganese dioxide to convert all of the iron dissolvedfrom the ore charge to the ferric state. After leaching for several hours enough limestone was added to the leach slurry to give it a pH of about 3.5 and thereby to cause the dissolved ferric iron to precipitate upon the leached residue. The leached residue was then separated from the pregnant leach liquor. To the pregnant leach liquor thus obtained there was added caustic calcined dolomite until a pH of 6.5 was attained. A precipitate was thus obtained which analyzed 4.39% in U308, 25% in A1203, 2% in Fe, 17.1% in SiOz, 7.5% in Ca0, 1.8% in Mn, in S04 and 2.68% in MgO.

A wet uranium containing hydroxide precipitate hav ing the assay set forth in the last sentence of the preceding paragraph was convertedto a slurry containing about 10% solids and having-a pH of 3.5 by the addition of sulfuric acid thereto. the added sulfuric acid at 90 C. for 16 hours. The temperature of 90 C. was maintained by boiling the slurry under reduced pressure.

The digest liquor was separated from the digested solid residue by filtration and was found to contain 99.11% of the uranium values that were originally present in the precipitate that was digested. The digested solid residue was assayed and from this assay it was found that 0.89% of the uranium, 55.8% of 'the aluminum, 93.2% of the silica, and 98.2% of the iron that were originally contained in the precipitate that was digested were retained in the digested solid residue. This showed how the digestion procedure of the invent-ion served to selectively extract the uranium values while it left the greater part of the aluminum, silica and iron with the digested solid residue.

If the low grade uranium containing hydroxide precipitates contain significant amounts of ferrous iron, the acid digest liquor obtained therefrom will contain much dissolved iron. In order to prevent this, precipitates which contain significant amounts of ferrous iron are pulped with water and then air-oxidized at pH 5.5 to 7 for from 2 to 24 hours in order to oxidize the ferrous iron therein to the ferric state. A small quantity of alkali may be added to neutralize the acid produced in this aeration step to keep the pH higher than 5.5. The oxidized uranium precipitate is separated by filtration from excess water before it is digested with sulfuric acid.

In digesting the low grade uranium containing hydroxide precipitates which also contain large amounts of hydrated silica and iron and aluminum hydroxides it is important to maintain the pH of the slurry being digested at from 3 to 3.8 in order that the digest liquor and di gested solid residue may be readily separated by filtration. When the pH is held at 3.2 to 3.5 the filterability of these digested slurries is at an optimum. The temperature of digestion may range from 68 C. to 100 C., but is preferably maintained at about 90 C. by boiling under reduced pressure. The acid slurry should be digested for at least 1 hour but best results are obtained if the digestion is continued for at least 16 hours.

In precipitating uranyl peroxide from the acid digest liquors, it has been found desirable to add enough H202 to the digest liquor so that the H202 will be present in the digest liquor to the extent of about 0.2 mol per liter of solution. In order to produce a granular and easily filterable form of the uranyl peroxide precipitate, it is best to precipitate the uranyl peroxide at a temperature of 25 C. to C. and at a pH of 2.5 to 3.5. The hydrogen peroxide used as a precipitating reagent generally is an aqueous solution containing from 19% to 30% of The slurry was then digested with r H202. The uranyl peroxide precipitates consist chiefly of uranyl peroxide but also contain minor amounts of aluminum, iron, calcium, magnesium, and manganese.

If desired, the uranium may be precipitated from these acid digest liquors by the addition of caustic calcined magnesia thereto. However, such a precipitate contains much larger amounts of iron, aluminum, and generally contains no more than 30% of uranium. Since H202 is a specific precipitant for uranium whereas MgO is not, it is preferred to use H202 to precipitate uranium from these acid digest liquors in order to obtain a precipitate which assays high in U308- In commercial embodiments of the present invention various liquors and solutions that contain small amounts of uranium are recycled instead of being run to waste as in individual batch experiments. For instance, the barren solution that remains after the uranyl peroxide precipitate has been separated and the wash solution that has been used to wash the acid digested residue may be combined and used in pulping up fresh ore that is to be leached with sulfuric acid in the presence of M1102. This recycling of these liquors not only conserves water employed in the process but also serves to cut down potential losses of uranium.

It has been found experimentally that it is quite necessary to digest the low grade uranium containing hydroxide precipitates which also contain large amounts of hydrated silica and iron and aluminum hydroxides with sulfuric acid at a pH in the range 3 to 3.8, at a temperature between 68 C. and 0., and for a period of at least one hour if adequate uranium extraction from these precipitates is to be obtained in a digest liquor which is easily separated by filtration from the digested residue and if the digested residue is to retain the major amount of the silica, iron, and aluminum which were originally associated with the low grade precipitate which was digested. It might be expected that almost any acid extraction method would be suitable for extracting the uranium values from these low grade uranium containing hydroxide precipitates. However, it has been found that this is not true. It was found that, if concentrated sulfuric acid was added to aqueous slurries of these low grade precipitates, the slurries became unfilterable, and therefore this method of extracting the uranium from these precipitates was necessarily abandoned. Attempts to leach uranium from these low grade precipitates by agitating them with cold sulfuric acid for periods of 16 to 20 hours also resulted in the formation of viscous gel-like slurries that would not pass through a filter. If the low grade precipitates were roasted at 300 C. to 600 C. for several hours, slurried in water and then agitated with sulfuric acid, the uranium extraction was impaired and the slurry had no better filtering characteristics than slurries obtained by the direct cold sulfuric acid leaching of wet low grade uranium precipitates. If the low grade uranium containing hydroxide precipitates were baked with sulfuric acid for 2 hours at ZOO-250 C. and then water leached a good extraction of uranium from a filterable residue was obtained. However, the use of this acid baking process was found to be impractical since it would necessitate the use of expensive acid resistant furnaces and entail a large expense for heating. Furthermore, the acid baking process made the aluminum extractable, and therefore this process was not as selective for uranium as desired. From the foregoing, it may be appreciated that applicants specific extraction process which forms the subject matter of this invention has unforeseen advantages that could not have been predicted.

Resort may be had to such modifications and variations as fall within the spirit of the invention and the scope of the appended claims.

I claim:

1. A process for selectively extracting the uranium values from a low grade uranium containing hydroxide precipitate which also contains large amounts of hydrated silica and iron and aluminum hydroxides which comprises digesting said precipitate with sulfuric acid'at a pH in the range of 3 to 3.8 at a'temperature between 68 C. and 100 C. for a period of at least one hour, and then separating the digest liquor from the digested solid residue.

2. A process for selectively recovering a high grade uranium concentrate from a low grade uranium containing hydroxide precipitate that also contains large amounts of hydrated silica and iron and aluminum hydroxides which comprises digesting said precipitate-with sulfuric acid at a pH in the range of 3 to 3.8 at a temperature between 68 C. and 100 C. for a period of at least one hour, separating the digest liquor from the digested solid residue, adding hydrogen peroxide to the digest liquor to cause the precipitation of uranyl peroxide, and then separating the precipitated uranyl peroxide from the digest liquor.

3. A process for selectively recovering a high grade uranium concentrate from a low grade uranium containing hydroxide precipitate that also contains large amounts of hydrated silica and iron and aluminum hydroxides which comprises aerating an aqueous slurry of said precipitate while maintaining the pH of the slurry between 5.5 and 7 in order to convert all ferrous iron in the precipitate to ferric iron, digesting said precipitate with sulfuric acid at a pH in the range of 3.2 to 3.5 at a temperature of about 90 C. for a period of at least one hour, separating the digest liquor from the digested solid residue, adding hydrogen peroxide to the digest liquor to cause the precipitation of uranyl peroxide, and

then separating the precipitated uranyl peroxide from the digest liquor. V

4. A process for selectively extracting the uranium values from a low grade uranium containing hydroxide precipitate which also contains large amounts of hydrated silica and iron and aluminum hydroxides which comprises digesting said precipitate with sulfuric acid at a pH in the range of 3.2 to 3.5 at a temperature of about C. for at least 16 hours, and separating the uranium containing digest liquor from the digested solid residue which contains the greater part of the silica, iron and aluminum that were originally associated with the precipitate that was digested.

5. A process as defined in claim 4 wherein the uranium values are selectively precipitated from the uranium containing digest liquor by the addition of hydrogen peroxide thereto.

6. That step in a process for selectively upgrading a low grade uranium containing hydroxide precipitate which also contains large amounts of hydrated silica and iron and aluminum hydroxides which comprises digesting said precipitate in aqueous suspension in a sulfuric acid solution at a pH between 3 and 3.8 and at a tem perature between 68 C. and C. for the purpose of improving the settling and filtering properties of the digested residue thereby obtained.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS FOR SELECTIVELY EXTRACTING THE UNRANIUM VALUES FROM A LOW GRADE URANIUM CONTAINING HYDROXIDE PRECIPITATE WHICH ALSO CONTAINS LARGE AMOUNTS OF HYDRATED SILICA AND IRON AND ALUMINUM HYDOXIDES WHICH COMPRISES DIGESTING AND PRECIPITATE WITH SULFURIC ACID AT A PH IN THE RANGE OF 3 TO 3.8 AT A TEMPERATURE BETWEEN 68*C. AND 100*C. FOR A PERIOD OF AT LEAST ONE HOUR, AND THEN SEPARATING THE DIGEST LIQUOR FROM THE DIGESTED SOLID RESIDUE.
 2. A PROCESS FOR SELECTIVELY RECOVERING A HIGH GRADE URANIUM CONCENTRATE FROM A LOW GRADE URANIUM CONTAINING HYDROXIDE PRECIPITATE THAT ALSO CONTAINS LARGE AMOUNTS OF HYDRATED SILICA AND IRON AND ALUMINUM HYDROXIDES WHICH COMPRISES DIGESTING SAID PRECIPITATE WITH SULFURIC ACID AT A PH IN THE RANGE OF 3 TO 3.8 AT A TEMPERATURE BETWEEN 68*C. AND 100*C. FOR A PERIOD OF AT LEAST ONE HOUR, SEPARATING THE DIGEST LIQUOR FROM THE DIGESTED SOLID RESIDUE, ADDING HYDROGEN PEROXIDE TO THE DIGESTED LIQUOR TO CAUSE THE PRECIPITATION OF URANYL PEROXIDE, AND THEN SEPARATING THE PRECIPITATED URANYL PEROXIDE FROM THE DIGEST LIQUOR. 